TRANSACTIONS OF SECTION C. 763 



a similar gradation in the mineral percentages, the felspar increasing from 12 per cent, 

 to 64 per cent., and the pyroxene diminishing from 67 per cent, to 10 per cent, in a 

 southerly direction. The author briefly stated that the contact metamorphism due 

 to these plutonic rocks was quite diS'erent in character from that produced by a 

 neighbouring mass of quartz-syenite in the same group of sedimentary rocks. 



The eruptive bosses are accompanied by a great series of dikes and sheets of 1am- 

 prophyric character, and varying from camptonite to bostonite. The author brought 

 forward a quantity of evidence to prove that (1) these two extreme types, with 

 SiO., percentages varying from 40 to 56, had been derived from the same magma ; 

 that (2) 9 parts of camptonite and 2 of bostonite (about the proportion observed 

 in the field) would give a magma of the composition of the Olivine-gabbro-diabase 

 of Solvsberget ; that (3) these lamprophyric dikes had been derived from the same 

 magma as the plutonic rocks ; and that (4) the diiferentiation had been efiected 

 while the magma still remained fluid. It was further shown that the differentiation 

 was probably due to the migration of less soluble constituents to the cooling 

 margin ; that the camptonites had a composition closely allied to that of the brown 

 hornblendes of the area ; and that, while the essential cooling of the plutonic rocks 

 had taken place in the eruptive bosses themselves, the dike rocks had segregated 

 before extrusion. 



A subsidiary diSerentiation of the plutonic rocks has also taken place in some 

 of the bosses, giving rise in the pure basic Brandberget to a pyroxenite (with 95 per 

 cent, pyroxene) and augite-diorite, and in the less basic Solvsberget pyroxenite and 

 labrador-porphyrite. 



Other points of information to be noted were : (1) That, under different physical 

 conditions, not only various mineral aggregates, but rocks of varying chemical 

 composition had resulted from the differentiation of the same magma ; (2) that 

 similar products result in this case from the segregation of an Olivine-gabbro- 

 diabase magma as have elsewhere been derived from a magma that has produced 

 nepheline-syenite ; (3) that the direction of segregation, according to laws of 

 crystalhsation, throws considerable light on the order of volcanic eruptions from 

 neighbouring centres. 



2. Petrological Features of the Dissected Volcano of Crandall Basin, 

 Wyoming. By Professor Joseph Paxson Iddings. 



It will not be possible in an abstract to do more than present in the briefest 

 manner the more sali?nt features of the petrology of the rocks of this volcano. The 

 rocks are mostly the same as those in various parts of the Yellowstone National Park, 

 some of which have been described in another place. The older acid breccia con- 

 sists of fragments and dust of hornblende-mica-andesite, hornblende-andesite, and 

 hornblende-pyroxene-andesite. They are partly glassy and partly holocrystalline. 

 In some places they appear to pass into the overlying breccia, but in others they 

 have been eroded and weathered before the latter were thrown over them. 



The upper breccia, which constitutes the main mass of the volcano, is basaltic 

 as a whole. It consists of pyroxene-andesite and basalt, the latter predominating 

 in the upper part of the accumulation. The massive flows, so far as investigated, 

 are all basalt. The composition varies constantly within narrow limits. A great 

 part of these rocks contain glassy ground mass. 



The rocks constituting the dikes exhibit more variation than the breccias, 

 though the majority of them are like the breccias in composition and habit, being 

 basalt. They are generally more crystalline. A great many dike rocks resemble 

 the basalts in outward appearance, but have no olivine, and are more crystalline. 

 The absence of olivine appears to be due to the conditions which influenced the 

 crystallisation of the rocks, and not to their chemical composition, for in some 

 cases what appear in hand specimens to be decomposed olivines are found to be 

 paramorpha after this mineral, consisting of grains of augite, magnetite, and biotite. 

 As the rocks become more crystalline biotite becomes an essential constituent, the 

 porphyritic minerals lose their sharpness of outline and assume some of the 

 microscopical characteristics they possess in gabbro. 



